The present invention relates to a deterioration diagnostic system of an exhaust gas purifying catalyst and more particularly to a diagnostic system including upstream and downstream air-fuel ratio sensors which are provided on an upstream side and a downstream side of an exhaust gas purifying catalyst.
An oxidization-reduction type exhaust gas purifying catalyst (hereinafter, referred to as a three-way catalyst) is provided in an exhaust system of an automotive gasoline engine (internal combustion engine) for decreasing harmful substances contained in exhaust gases.
In this three-way catalyst, while a chemical reaction of oxidizing the harmful substances such as hydrocarbons (HC) and carbon monoxide (CO) occurs, a chemical reaction of reducing oxides of nitrogen (NOx) occurs, whereby exhaust gases are purified.
Air-fuel ratio sensors (oxygen sensors) are provided on an upstream side and a downstream side of the three-way catalyst, respectively, and the deterioration of the three-way catalyst is diagnosed based on output signals from these oxygen sensors (refer, for example, to JP-A-8-218853.).
In a deterioration diagnostic system of an exhaust gas purifying catalyst which is described in JP-A-8-218853, a hysteresis constant α is added to or subtracted from a mean value VORave calculated based on an output voltage VOR of the downstream O2 sensor to calculate upper and lower inversion or switch thresholds THH, THL, and a downstream side switch frequency fR is calculated from the number of times the output voltage VOR crosses the switch thresholds THH, THL, while an upstream side switch frequency fF is calculated from the number of times an output voltage VOF of the upstream O2 sensor crosses its threshold VTH. Then, a switch frequency ratio fR/fF is calculated from the upstream side switch frequency fF and the downstream side switch frequency, and it is determined that the three-way catalyst is deteriorating in the event that the switch frequency ratio fR/fF is in excess of a predetermined value THc.
In the above-mentioned deterioration diagnostic system of the exhaust gas purifying catalyst, it is possible to detect the deterioration of the catalyst, however, it is difficult to determine the deterioration of the catalyst with high accuracy.
Namely, in the deterioration diagnostic system of the exhaust gas purifying catalyst which is described in JP-A-8-218853, when the accuracy with which the deterioration of the catalyst is determined is increased by setting small the upper and lower switch thresholds THH, THL so as to make them close to the mean value VORave, the correction amount of air-fuel ratio and feedback becomes excessive as the accuracy so increases due to the deterioration of the upstream O2 sensor even in the event that the catalyst is not deteriorated. Since this increases the amplitude of the air-fuel ratio and causes the output signals from the upstream and downstream O2 sensors to synchronize to thereby increase the switch frequency ratio fR/fF, there has been a possibility that it is erroneously determined that the catalyst is deteriorating. In particular, in the event that the intake air volume is large, the exhaust gas purifying reaction time in the catalyst becomes short and this makes it difficult to absorb the amplitude of the air-fuel ratio, whereby the risk that the erroneous determination is made becomes high.